In the combustion of coal, such as for power generation and/or steam heating, there are generated flue gases which contain sulfur compounds, principally sulfur dioxide. Coal generally is classified as low, medium or high sulfur coal, depending upon the quantity of sulfur contained within a unit quantity of the coal, i.e. coal having between about 400 to 500 ppm of sulfur is deemed low-sulfur coal and coal having between about 1000 to 3000 ppm or higher of sulfur is deemed high-sulfur coal. All coals thus contain some sulfur and upon combustion of the coal there is released into the flue gases gaseous sulfur dioxide. In those coal-burning units where low or medium-sulfur coal is burned, it has been common heretofore to convert such gaseous sulfur dioxide to solid calcium sulfites/sulfates and to thereafter capture such solid particulates employing any of several well-known particulate removal systems. In one common system for such conversion, the hot flue gas, containing the gaseous sulfur dioxide, is introduced into a spray dryer along with an atomized slurry of calcium hydroxide (Ca(OH).sub.2). In the spray dryer, the calcium hydroxide and sulfur dioxide, in the presence of the water, react to form various calcium sulfite and/or sulfate solid particulates which are subsequently collected, e.g. by particulate removal techniques using fabric filters and/or electrostatic precipitators.
In this technique for removal of the sulfur dioxide from the flue gases, it is a major function of the spray dryer to cool the gaseous effluent (and the solids suspended therein) to a temperature as reasonably near the water saturation temperature for the gaseous effluent as possible (i.e. in the range of about 130.degree. to 140.degree. F.) to permit the sulfur dioxide to be captured as efficiently as possible as is commonly understood in spray dryer technology. These prior art systems function quite satisfactorily when processing the flue gas from low or medium sulfur coal combustion. The prior art Ca(OH).sub.2 -based spray dryer/fabric filter systems are capable of removing substantially all the sulfur dioxide from the flue gas (90% or greater). However, such prior art systems are incapable of removing all of the sulfur dioxide from the flue gas emanating from the combustion of high sulfur coal. In this regard, it is noted that in such systems, the quantity of sulfur that can be removed per unit of flue gas is a function of the quantity of calcium hydroxide and water that can be fed into the spray dryer per unit of flue gas and still not cause the flue gas to be cooled to the water saturation temperature of the gas. Thus in these prior art systems, it is physically impossible to introduce into the spray dryer (1) sufficient calcium hydroxide slurry as will provide the quantity of calcium that is necessary to react with all the sulfur in the flue gases emanating from burners of high sulfur coal as well as (2) the quantity of water required to cool the effluent from the spray dryer to a temperature approaching the water saturation temperature of such effluent. Any unreacted sulfur, of course, escapes through the particulate control system to the atmosphere as a contaminant.
Still further, it is known in the prior art that the efficiency of utilization of the calcium hydroxide decreases at the higher slurry concentrations such as are required to provide sufficient calcium for approaching complete reaction with all the sulfur present in the flue gasses.